Accurately determining the number of Auger electrons per nuclear decay for medical isotopes

Abstract

Auger-emitting radionuclides have great potential in cancer treatment as low-energy Auger electrons (below 5 keV) deposit a high radiation dose over a very short distance. Auger-based therapy is attractive as, in contrast to alpha and beta particles, the affected volume around the decaying radionuclides is much smaller than the volume of a cell. Moreover, low-energy electrons (3-20 eV) have been shown to be very effective in causing DNA cleavage. In this study, a monolayer of 125I was deposited on a gold surface and the energy of the emerging electrons was measured. 125I decays by electron capture to a nuclear excited state of 125Te. A core hole, which is created in the electron capture, relaxes via an Auger cascade that produces multiple Auger electrons in the process. Subsequently, the nuclear excited state of 125Te internally decays to the nuclear ground state and the excess energy can also eject an atomic electron (the conversion electron), which also leaves a core hole behind, and the newly created core hole will relax by an Auger cascade as well. The conversion electron and the Auger electrons have similar energies, and thus their intensities can be compared, linking the nuclear- and atomic-physics parts of the decay of 125I. A Monte Carlo model (BrIccEmis) was used to simulate the spectrum. BrIccEmis is based on the known intensity of the conversion electron line and estimates of the decay rates for Auger and X-ray emission. In order to fit the spectrum one has to assume a line shape model describing among others, the effects of shake off. We discuss whether one can get a good description of the spectrum of 125I absorbed on Au, based on the calculated one for atomic 125I decay with reasonable assumptions for the line shape. The measurements of the very low-energy Auger electrons (below 1 keV), which are particularly relevant to medical physics applications, are even more difficult to quantify, and preliminary results in this energy range are also discussed.